In the field of nucleic acid-related substances, there are known many substances having an antiviral activity or a carcinostatic activity, and some of them are clinically used as useful medical drugs. For example, as antiviral agent, vidarabine [M. Privat de Garilhe and J. de Rubber, C. R. Acad. Soc. D (Paris) 259, 2725 (1964)], aciclovil [G. B. Elion et al., Proc. Natl. Acad. Sci, USA, 74, 5716 (1977)], azidothymidine [H. Mitsuya et al., Proc. Natl. Acad. Sci. USA, 82, 7096 (1985)], etc. are known. As carcinostatic agent, 5-fluorouracil, cytosine arabinoside, etc. are known.
Apart from the above, it is also known that nucleic acid derivatives represented by the following formula have an antiviral activity [Journal of Antibiotics (J.A.) Vol. XLII, No. 12, 1989, pp. 1854-1859; Antimicrob. Agents Chemother. Vol. 32, No. 7, 1988, pp. 1053-1056; J.A., Vol. XLII, No. 4, 1989, pp 644-646]: ##STR2## wherein B' represents adenine or guanine and X represents oxygen atom or methylene.
It is an object of this invention to develop novel nucleic acid derivatives which are expected to have an antiviral activity.